This application is based on and incorporates herein by reference Japanese Patent Application No. 2000-93770 filed on Mar. 30, 2000.
1. Field of the Invention
The present invention relates to a geared motor that outputs a rotational force generated by a motor unit thereof through a worm gear assembly.
2. Description of Related Art
A geared motor has been used, for example, in a power window system of a vehicle. The geared motor generally includes a motor unit and a gear system connected to the motor unit. In a case of the power window system, a worm gear assembly is used as the gear system.
As shown in FIG. 7, one previously proposed geared motor 51 includes a cup-shaped yoke 52 that has a base wall at one end and an opening at the other end. The geared motor 51 further includes a gear housing 53 that covers the opening of the yoke 52. The yoke 52 receives a motor unit including a rotor, magnets, brushes and the like. The gear housing 53 receives a worm gear assembly including a worm, a worm wheel and the like. The gear housing 53 has a wheel housing segment 54 in which the worm wheel is received. An output shaft 55 is received and rotatably supported within a circular through hole that penetrates through a center of a shaft supporting portion arranged at a center of the wheel housing segment 54.
In the above geared motor 51, a rotational force of the motor unit is transmitted to the output shaft 55 via the worm gear assembly to output the rotational force from the geared motor. Then, the rotational force of the output shaft is transmitted to the power window system to drive the same.
The gear housing 53 is molded from a resin material. During a molding process of the gear housing 53, the wheel housing segment 54 may be warped when the resin material is cooled and solidified. Warping of the resin material of the wheel housing segment 54 causes the circular cross section of the through hole of the shaft supporting portion to be deformed into an ellipsoidal shape. The ellipsoidal through hole of shaft supporting portion causes an increase in a frictional resistance between the shaft supporting portion of the wheel housing segment 54 and the output shaft 55, so that the output shaft 55 cannot be freely rotated.
The present invention addresses the above described disadvantage. Therefore, it is an objective of the present invention to provide a geared motor that restrains warping of a gear housing during a molding process of the gear housing.
To achieve the objective of the present invention, there is provided a geared motor including a yoke and a gear housing. The yoke has an opening and receives a motor unit. The gear housing is made of a resin material. The gear housing covers the opening of the yoke and receives a worm gear assembly for transmitting a rotational force of the motor unit to an output shaft connected to the worm gear assembly. The worm gear assembly includes a worm wheel. The gear housing has a wheel housing segment that receives and rotatably supports the worm wheel. The wheel housing segment has a base wall. The output shaft is connected to the worm wheel and is rotatably received in the base wall of the wheel housing segment such that an axial direction of the output shaft is generally perpendicular to a plane of the base wall of the wheel housing segment. The geared motor further includes a plurality of ribs extending over at least part of an outer surface of the base wall of the wheel housing segment. Each one of the ribs has a lateral thickness that is measured in a direction perpendicular to the axial direction of the output shaft and that is equal to or smaller than an axial thickness of the base wall of the wheel housing segment measured in the axial direction of the output shaft.